Dynamic laser marking

ABSTRACT

A method and an apparatus for making a moving body of material (26). The method includes the steps of directing at the moving body a high energy density beam (46,58), concentrating the beam so as to produce an illuminated spot at a location on or within the moving body, and moving the spot in accordance with the resultant of two components of movement, the first component being equal to the velocity of the moving body and the second component being relative to the moving body, so as to create a mark of a predetermined shape. In a preferred embodiment, the apparatus includes at least one movable galvanometer mirror (68,70) capable of moving the spot in accordance with the resultant of the two components.

The present invention relates to a method and an apparatus for marking amoving body of material using a high energy density beam.

Many products are manufactured or processed on production lines with theproduct concerned moving continuously from one work station to anotheruntil all the manufacturing or processing steps have been completed.Often the marking of the product is incorporated into the productionline, establishing a requirement for a device capable of marking theproduct without adversely affecting the continuous movement of theproduction line.

One such device in use today is the ink jet marker which is capable ofdirecting a controlled jet of ink onto a moving package so as to producea desired indicum. Such devices are capable of marking up to 1000 itemsa minute but require constant attention and frequent overhaul to preventthe nozzle of the ink jet from fouling. Such an overhaul may necessitatethe shutting down of the production line, with a consequent loss inmanufacturing or processing time. Furthermore, devices of this typeconsume a large quanitity of materials such as ink and solvent,resulting in them having a significant running cost. Questions have alsobeen raised as to the indelibility of the resulting mark.

Laser marking on the other hand, offers a clean and elegent alternativeto ink jet marking and provides the body concerned with a truelyindelible mark.

Broadly speaking, current commercial laser marking techniques fallwithin one of two categories. In the first of these categories a beam ofunfocused laser radiation is passed through a mask so as to produce thedesired pattern, while in the second a beam of laser radiation isscanned across the object concerned, tracing out the desired pattern.

U.S. Pat. No. 4,758,703 provides an example of a marking techniquefalling into the first category, and it describes a method of covertlyencoding a microscopically visible pattern onto the surface of a movingobject. In the method described, the presence of a moving object issensed and the speed of its approach measured so that at the appropriatemoment, when the object passes the laser head, a beam of unfocused laserradiation is directed onto the object through a mask. It is the maskthat is responsible for generating the pattern of the marking, and itcomprises a mask plate having a cross-sectional area greater than thatof the beam and incorporating a matrix of holes which may or may not beobscured. Having passed through the mask, the beam is focused to reducethe size of the pattern produced on the surface of the package, as wellas to increase the intensity of the beam. In the particular methoddescribed, the intensity of the beam is carefully controlled so that thefinal pattern is barely etched on to the surface and remains invisibleto the naked eye.

The Applicant's own co-pending UK Patent Application No. 9117521.6provides an example of a scanning method of laser marking and relates toa method and apparatus for providing a body of material with asub-surface mark in the form of an area of increased opacity toelectromagnetic radiation. The method comprises the steps of directingat a surface of the body a high energy density beam to which thematerial is transparent and bringing the beam to a focus at a locationspaced from the surface and within the body so as to cause the localisedionisation of the material. UK Patent Application No. 9117521.6additionally relates to a body marked in accordance with the said methodor by use of the said apparatus.

Although the scanning laser marking technique has the advantage of beingmore flexible in that the shape of the desired mark may be changedexternally without interrupting the operation of the laser to change amask element, the technique has yet to be used commercially for markingmoving bodies because of fears that the resulting mark would be blurredor else "Stretched" in the direction of motion of the body. This fearhas to date confined the scanning laser marking technique toapplications in which the body to be marked is stationary, leavingmoving bodies to be marked using the masked beam technique, although theclarity of the resulting mark using this technique is also ultimatelylimited by the speed of movement of the moving body.

According to a first aspect of the present invention, there is provideda method of marking a moving body of material comprising the steps ofdirecting at the moving body a high energy density beam, concentratingthe beam so as to produce an illuminated spot at a location on or withinthe moving body, and moving said spot in accordance with the resultantof two components of movement, the first being equal to the velocity ofthe moving body and the second being relative to the moving body, so asto create a mark of a predetermined shape.

In a preferred embodiment, there is included the additional step ofdetermining the velocity of the moving body. While it is recognised thatthe velocity of the moving body may be determined by monitoring thespeed of movement of the means used to transport the body, the velocityof the moving body is preferably determined by means of directmeasurement.

Advantageously, the high energy density beam is directed at the movingbody by causing the path of the moving body to intersect the path of theactuated high energy density beam, and acutating the high energy densitybeam at a predetermined time after the moving body passes a position aknown distance from the point of intersection, that time being dependantupon the velocity of the moving body.

In a particular embodiment, wherein the mark comprises a sub-surfacemark, the high energy density beam is preferably brought to a focus at alocation within the moving body so as to cause localised ionisation ofthe material and the creation of a mark in the form of an area ofincreased opacity to electromagnetic radiation. In such an embodiment,the moving body of material may be transparent to electromagneticradiation at wavelengths within the visible region, thereby renderingthe mark visible to the naked eye. For example, the material may be ofglass or plastic. Alternatively, the moving body of material may beopaque to electromagnetic radiation at wavelengths within the visibleregion so that the mark may only be "seen" by optical instrumentsoperating at an appropriate wavelength within the electromaganeticspectrum. While such a mark is not capable of performing many of thefunctions of its visible counterpart, it does represent a trulyindelible covert mark.

In this or any other embodiment, the mark may comprise one or morenumerals, letters or symbols, or a combination thereof, which in turnmay represent an indentification, a trade mark, a machine readable codeor any other desired indicum. In addition, the mark may be threedimensional.

According to a second aspect of the present invention, there is providedan apparatus for marking a moving body of material comprising means forcreating a high energy density beam and directing the beam at the movingbody, means for concentrating the beam so as to produce an illuminatedspot at a location on or within the moving body, and means for movingsaid spot in accordance with the resultant of two components ofmovement, the first being equal to the velocity of the moving body andthe second being relative to the moving body, so as to create a mark ofa predetermined shape

Advantageously, the means for moving the spot in accordance with theresultant of two components includes means for moving said spot inaccordance with the said second of the two components, the meansperferably including at least one moveable mirror disposed in the pathof the beam. The movement of the mirror may be controlled in accordancewith a computer program enabling the final shape of the mark to beeasily manipulated, while the moveable mirror itself may comprise agalvanometer mirror. While it is recognised that any suitable means maybe provided to move the mirror, such as a servo motor or manual joystick, the properties of a galvanometer mirror provide a speed ofresponse and an ease of control that represent a significant advantageover alternative control means.

In a preferred embodiment, the means for moving the spot in accordancewith the said second of the two components is also capable of moving thespot in accordance with the said first of the two components.

In another embodiment, the means for moving said spot in accordance withthe resultant of two components includes additional means for moving thespot in accordance with the said first of the two components, the meanspreferably including at least one rotatably mounted mirror whose speedof rotation is varied in accordance with the velocity of the movingbody.

In a further embodiment, the rotatably mounted mirror of the precedingembodiment is multi-faceted.

In yet a further embodiment, the means for moving the spot in accordancewith the said first of the two components includes at least one mirrormoveable at the same velocity as the moving body.

In yet a further embodiment, the means for moving the spot in accordancewith the said first of the two components includes at least oneacusto-optic or elctro-optic crystal.

In a preferred embodiment, there is further provided means fordetermining the velocity of the moving body. While it is recognised thatthe velocity of the moving body may be determined by monitoring thespeed of movement of the means used to transport the body, the velocityof the moving body is preferably determined by means of directmeasurement. For example, in a particular arrangement, the velocity ofthe moving body may be determined by measuring the time taken for themoving body to travel between two opto-detectors spaced a known distanceapart.

Advantageously, the high energy density beam is directed at the movingbody by causing the path of the moving body to intersect the path of thehigh energy beam when actuated, and providing means to actuate the highenergy density beam at a predetermined time after the moving body passesa position a known distance from the point of intersection, that timebeing dependant upon the velocity of the moving body.

The means for concentrating the beam may include a lens element having afocal length that varies across its width so as to compensate for aparticular de-focusing effect. Alternatively, or in addition, the meansfor concentrating the beam may include a zoom lens to either againcompensate for a particular de-focusing effect or to enable marks to bemade at different depths within the body and so allow for the creationof three dimensional marks. In a particular arrangement, the means forconcentrating the beam may include a diverging lens.

In a particular embodiment, wherein the mark comprises a surface mark,the means for creating a high energy density beam preferably comprises aCO₂ laser.

In an embodiment wherein the mark comprises a sub-surface mark, themeans for creating a high energy density beam preferably comprises alaser which is focused so as to have a peak energy density at the focusof a least 10 J/cm². This peak energy density is preferably achieved bymeans of a laser which is focused to have a power density at the focusof at least 10⁷ W/cm² and is pulsed with a pulse duration of at least10⁻⁶ seconds. If in such circumstances, the body of material to bemarked is transparent to electromagnetic radiation at wavelengths withinthe visible region, then the means for creating the required high energydensity beam is preferably a Nd-YAG (Neodymium-doped Yittrium AluminiumGarnet) laser operating at a wavelength of 1.06 μm.

Advantageously, a secondary source of visible laser radiation may beprovided to facilitate alignment of the high energy density beam.

A conveyor belt is preferably provided to transport the moving body, andin such circumstances the conveyor belt may be provided with means tocontrol the lateral position of the moving body relative thereto.

A number of embodiments of the present invention will now be described,by way of example, with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic plan view of a laser marking station in accordancewith the second aspect of the present invention in which a markingapparatus and sensing module are shown disposed adjacent a continuouslymoving conveyor belt;

FIG. 2 is a schematic diagram of the sensing module of FIG. 1;

FIG. 3 is a schematic diagram of the marking apparatus of FIG. 1;

FIG. 4 is a schematic diagram of the way in which electrical power isdistributed throughout the marking appartaus of FIG. 1;

FIG. 5 is a flow diagram of the combined operational sequence of themarking apparatus and sensing module of FIG. 1;

FIG. 6 is schematic diagram of a marking apparatus in accordance with asecond embodiment;

FIG. 7 is a schematic diagram of a marking apparatus in accordance witha third embodiment; and

FIG. 8 is a schematic diagram of a marking apparatus in accordance witha fourth embodiment; and

FIG. 9 is a schematic diagram of a marking apparatus in accordance witha fifth embodiment.

The laser marking station shown in FIG. 1 comprises a marking apparatus10 and a sensing module 12, both of which are contained within aprotective housing 14 that straddles a continuously moving conveyor belt16.

The conveyor belt 16 is generally of sufficient width to transport tothe laser marking station a body of material to be marked and is furtherdefined by a moving web 18 and two vertically projecting side rails 20and 22. Typically the first of the two side rails 20 is fixed relativeto the moving web 18, while the other side rail 22 is moveable relativethereto by means of an adjusting screw 24. Upon tightening the adjustingscrew 24, the distance between the two side rails 20 and 22 isdecreased, thereby narowing the effective width of the conveyor belt 16.

The body of material to be marked, which in the accompanying drawings isdepicted as a glass bottle 26, is transported to the laser markingstation by the conveyor belt 16 and enters the protective housing 14 bya first opening 28. Thereafter the body 26 is transported past thesensing module 12 and the marking apparatus 10 before passing out of theprotective housing 14 through a second opening 30. For the purposes ofsafety, the distance between the marking apparatus 10 and either of thefirst or second openings 28 or 30 is such as to ensure that an operatorcan not accidentally reach inside the protective housing 14 and placehis or her hand in front of the marking apparatus 10.

The sensing module 12 is shown in greater detail in FIG. 2 and comprisesa pair of opto-detectors 32 and 34 disposed side by side adjacent theconveyor belt 16. Each of the opto-detectors 32 and 34 includes a lightsource 36 and a suitable detector 38 and is aligned with one of a pairof respective retro-reflectors 40 or 42 disposed on the opposite side ofthe conveyor belt 16. Light is emitted from the light source 36 towardthe associated retro-reflector, whereupon it is reflected back towardthe opto-detector and detected by the detector 38. Thus, when there isnothing between an opto-detector and its corresponding retro-reflector,as shown in relation to opto-detector 34 in FIG. 2, the quantity oflight detected by the detector 38 is a maximum. However when the opticalpath between an opto-detector and the its corresponding retro-reflectoris obstructed by, for example, the passage of the body to be markedalong the conveyor belt 16 as shown in FIG. 2 in relation toopto-detector 32, then the quantity of light reflected by thecorresponding retro-reflector, in this case retro-reflector 40, anddetected by the detector 38 falls below a pre-set threshold value, andan appropriate signal is generated.

In order to enhance the sensitivity of each of the opto-detectors 32 and34, the light source 36 is selected so as to emit light in the visibleor near infra-red region of the elctromagnetic spectrum, while thedetector 38 is chosen not only to be selectively sensitive to thisparticular frequency range, but also to be responsive only to lighthaving the polarisation characteristics of the source 36. In this waythe detector 38 is insensitive to light emanating from sources otherthan the light source 36 or to light reflected from surfaces other thanthe associated retro-reflector, such as, for example, from the surfaceof the body to be marked, since such reflections would typically possessdifferent polarisation characteristics.

The marking apparatus 10 is shown in greater detail in FIG. 3 andcomprises a source 44 of laser radiation 46 which is directed so as tointersect the path of the moving body 26.

In a first embodiment, the marking appartus 10 is designed so as tofacilitate the surface marking of the moving body 26. To this end, laserradiation of a sufficient energy density is directed toward the body 26,causing areas of the surface impinged thereby to melt and reflow,leaving a resulting mark. In the particular embodiment illustrated inFIG. 3, the source 44 comprises an RF excited simulated continuous-wavecarbon dioxide (CO₂) laser that emits a beam of laser radiation 46having a wavelength of 10.6 μm and which is consequently invisible tothe naked eye. Having been emitted from the CO₂ laser 44, the beam oflaser radiation 46 is incident upon a first reflecting surface 48 thatdirects the beam 46 through a beam expander 50 and a beam combiner 52 toa second reflecting surface 54. A second source of laser radiation, inthe form of a low power He-Ne (Helium--Neon) laser 56, is disposedadjacent to the CO₂ laser 44 and emits a secondary beam of visible laserradiation 58 with a wavelength of 638 nm. The secondary beam 58 impingesupon the beam combiner 52 where it is reflected toward the secondreflecting surface 54 coincident with the beam of laser radiation 46from the CO₂ laser 44. Thus, the necessary properties of the beamcombiner 52 are that it should transmit electromagnetic radiation with awavelength of 10.6 μm while reflecting electromagnetic radiation with awavelength of 638 nm. In this way the He-Ne laser beam 58 provides thecombined CO₂ /He-Ne beam 46, 58 with a visible component thatfacilitates optical alignment.

Once combined, the two coincident beams 46,58 are reflected at thesecond reflecting surface 54 toward a third reflecting surface 60, andfrom the third reflecting surface 60 are further reflected toward afourth reflecting surface 62. From the fourth reflecting surface 62, thecombined beam 46, 58 is reflected yet again toward a head unit 64 fromwhich the combined beam 46, 58 is finally directed so as to intersectthe path of the moving body 26. In order to faciliate marking atdifferent heights from the base of the body 26, the third and fourthreflecting surfaces 60 and 62 are integrally mounted, together with thehead unit 64, so as to be adjustable in a vertical plane under theaction of a stepping motor 66 (FIG. 4).

Within the head unit 64 the combined CO₂ /He-Ne beam 46, 58 issequentially incident upon two moveable mirrors 68 and 70. The first ofthe two mirrors 68 is disposed so as to be inclined to combined beam 46,58 that is incident upon it as a result of reflection from the fourthreflecting surface 62 and is moveable in such a way as to cause the beamreflected therefrom to move in a vertical plane. The second of the twomirrors 70 is similarly inclined, this time to the beam 46, 58 that isincident upon it as a result of reflection from the first mirror 68, andis moveable in such a way as to cause the reflected beam 46, 58 to movein a horizontal plane. Consequently it will be apparant to those skilledin the art that the beam 46, 58 emerging from the head unit 64 may bemoved in any desired direction by the simultaneous movement of the firstand second mirrors 68 and 70. In order to facilitate this movement, thetwo moveable mirrors 68 and 70 are mounted on respective first andsecond galvanometers 72 and 74. While it is recognised that any suitablemeans maybe provided to control the movement of the two mirrors 68 and70, such as individual servo motors or a manual joystick, the approachadopted combines a speed of response with an ease of control thatrepresents a significant advantage over alternative control means.

Emerging from the head unit 64, the combined beam 46, 58 is focused bypassing through a lens assembly 76 which may include one or more lenselements. A first lens element 78 is capable of bringing the beam 46, 58to a focus, at a chosen location on the surface of the body to bemarked. As is well known, the maximum power density of the beam 46, 58is inversely proportional to the square of the radius of the beam 46, 58at its focus, which in turn is inversely proportional to the radius ofthe beam 46, 58 that is incident upon the focusing lens 78. Thus, for abeam 46, 58 Of electromagnetic radiation of wavelength λ and radius Rthat is incident upon a lens of focal length f, the power density E atthe focus is, to a first approximation, given by the expression:##EQU1## Where P is the power produced by the laser. From thisexpression the value and purpose of the beam expander 50 is readilyapparent since increasing the radius R of the beam serves to increasethe power density E at the focus. In addition, the lens element 78 istypically a short focal length lens having a focal length in the rangebetween 70 mm and 80 mm, so that typical power densities at the focus ofthe beam 46, 58 are in excess of 300 W/cm². At power densities of thisorder, thermal interactions occur at the surface of the body to bemarked 26 in which the incident radiation 46, 58 is absorbed as heat.This localised heating causes the surface of the body 26 to melt andreflow, leaving a residual mark inscribed upon the surface. By movingthe focus of the beam 46, 58 using the mirrors 68 and 70, the mark maybe made to a predetermined shape, and in particular, may be made tocomprise one or more numerals, letters or symbols, or a combinationthereof, which in turn may represent an identification, a trade mark, amachine readable code, or any other desired indicum.

The power density required to stimulate thermal interactions at thesurface of the body will, of course, depend upon the material of thebody and the speed at which the beam 46, 58 is scanned. Materials suchas perspex may be marked using a beam 46, 58 having a power density ofas little as approximately 50 W/cm², while to mark some metals it isnecessary for the beam 46, 58 to have a power density approximately 1MW/cm². Bodies made of glass fall between these two extremes and may bemarked using a beam 46, 58 having a power density of in excess of 300W/cm² and a scanning speed of 3 m/sec.

In the interests of safety, the two lasers 44 and 56 and theirrespective beams 46 and 58 are enclosed within a safety chamber 80 asshown in FIG. 4, with the combined beam 46, 58 emerging from the safetychamber 80 only after passing through the lens assembly 76. Access tothe two lasers 44 and 56 and the various optical elements disposed inthe path of the respective beams 46, 58 is gained by means of a doorpanel 82 which is fitted with an interlock 84 that prevents theoperation of the CO₂ laser 44 while the door panel 82 is open. It is tobe noted that the He-Ne laser 56 need not necessarily be fitted with aninterlock in the same way, since it only operates at a very low powerand does not represent a significant danger to a skilled operator.

A single phase electrical mains supply of 240 V is fed via the doorpanel interlock 84 to a mains distribution unit 86 that is disposedbelow, and isolated from, the safety chamber 80 in order to prevent anyelectrical effects interfering with the operation of the lasers 44 and56. From the distribution unit 86, mains electrical power is provided tothe CO₂ laser 44 and He-Ne laser 56, as well as to a chiller unit 88that serves to cool the CO₂ laser 44. In addition, mains electricalpower is also supplied to the stepping motor 66 and to a computer 90.Three AC/DC convertors and associated voltage regulators provideregulated DC voltage supplies of 9 v, 12 v and 15 v that are fedrespectively to the He-Ne laser 56 to facilitate the pumping mechanism,to a further interlock 92 that prevents premature firing of the CO₂laser 44, and to the head unit 64, and in particular to the first andsecond galvanometers 72 and 74, to produce a predetermined movement ofthe first and second mirrors 68 and 70.

The combined operational sequence of the marking appratus 10 and thesensing module 12 is shown schematically in FIG. 5 and commences withthe computer 90 either calculating or performing a look-up to identifythe next mark to be applied. Thus, if the laser marking station is usedto mark a number of bodies, each with a sequential reference number, thecomputer 90 may calculate the next mark by adding the necessaryincrement to the reference number that constitued the previous mark.Alternatively, at the start of a batch, or during more complicatedmarking sequences, the computer 90 may identify the next mark from oneof a pre-programmed list of marks contained in a suitable memory device.However the next mark is identified, it may be displayed on anoperator's console along with other information, such as the number ofbodies marked in a particular batch, the average line speed of thebodies transported past the sensing module 12 and any other desiredinformation.

Having identified the mark to be applied to the moving body 26, thecomputer 90 calculates the vectors necessary to trace out the markassuming the body 26 to be stationary at the time of marking. Thesevectors are transformed into an electrical signal that, if used tomodulate the 15 v DC supply applied to the first and secondgalvanometers 72 and 74, would produce a series of movements of thefirst and second mirrors 68 and 70 capable of moving the focus of anactuated laser beam in such a way as to trace out the desired mark.

As the body to be marked is transported to the laser marking station bymeans of the conveyor belt 16 the position of the moving body 26relative to the fixed side rail 20 may be altered by means of adjustingscrew 24. Typically, adjusting screw 24 is used to narrow the effectivewidth of the conveyor belt 16 adjacent the first opening 28 in theprotective housing 14. In this way the effective width of the conveyorbelt 16 is made not much wider than the moving body 26 itself, therebyproviding a degree of control over the lateral distance between the bodyto be marked and the various components of the sensing module 12 and themarking apparatus 10.

All this time the sensing module 12 is used to detect the approach ofthe body to be marked. As the body 26 reaches opto-detector 32 itsleading-edge obstructs the optical path between the light source 36, theretro-reflector 40 and the detector 38, causing the quantity of lightdetected to fall below a preset threshold value. As a result anapproriate signal is generated and sent to the computer 90, whereupon aclock is actuated. This clock is not stopped until a time t₁ later whenthe leading-edge of the moving body 26 is detected in the same way atthe second opto-detector 34. Since the two opto-detectors 32 and 34 area known distance d₁ apart, the velocity v of the body to be marked maybe readily calculated by dividing the known distance d₁ by the time t₁measured by the clock. Thus:

    v=d.sub.1 /t.sub.1

In order to provide a compact apparatus capable of marking bodies movingat relatively high line speeds, the distance d₁ between the twoopto-detectors 32 and 34 is preferably made as small as possible. In thelimiting case, opto-detector 34 is caused to abutt opto-detector 32,enabling d₁ to be reduced to a value of 1 mm. Even at such smalldistances, the oscillator that forms the basis of the clock is capableof in excess of 5 clock cycles during a typical time interval t₁ so thatthe reduction in d₁ does not have a discernable effect on the accuracyto which the velocity v maybe measured.

Having passed the second opto-detector 34, the body to be markedcontinues to be transported by the conveyor belt 16 until at a time t₂later it is adjacent the marking apparatus 10. Since the secondopto-detector 34 and the marking apparatus 10 are again a known distanced₂ apart, the time t₂ maybe calculated by dividing the distance d₂ bythe velocity v of the moving body 26. Thus:

    t.sub.2 =d.sub.2 /v

Or:

    t.sub.2 =d.sub.2 /d.sub.1 ×t.sub.1

Again, in order of provide a compact apparatus, the distance d₂ maybedecreased to a minimum, ultimately limited by the computing power of thecomputer 90, but which is typically of the order of 5 mm.

Using the above equation, the computer 90 calculates the estimated timeof arrival t₂ of the body to be marked adjacent the marking apparatus10. This time interval however represents the time at which theleading-edge of the body 26 is adjacent the marking apparatus 10, andso, unless the desired mark is to be applied at the leading-edge, afurther delay δt is added to the time interval t ₂ to yield a time t₃ atwhich that portion of the body 26 to be marked is adjacent the markingapparatus 10.

At a time t₃ after the signal generated by the second opto-detector 34,the CO₂ laser 44 is actuated and the combined CO₂ /He-Ne beam 46, 58focused at the estimated position of the surface of the body 26. At thesame time an electrical signal is generated to modulate the 15 v DCsupply applied to the first and second galvanometers 72 and 74 that notonly reproduces the vectors necessary to trace out the desired mark butalso includes a superimposed component that compensates for the movementof the body 26 at the velocity v. The modulated 15 v DC supply producesa series of movements of the first and second mirrors 68 and 70 thatdirect the focus of the combined CO₂ /He-Ne beam 46, 58 so as to traceout the desired mark, while at the same time moving the mark as it isbeing traced with a velocity v, thereby enabling real time dynamicscanning to take place.

After the body 26 has been marked it continues to be transported by theconveyor belt 16 and passes out the protective housing 14 and away fromthe laser marking station by way of the second opening 30. The markedbody 26 may thereafter be transported to further processing stations ifrequired, while the computer 90 calculates the next mark to be appliedand the operational sequence begins all over again.

It will be apparent to those skilled in the art that as the body 26moves past the marking apparatus 10, the distance between the lensassembly 76 and that portion of the surface of the body 26 that is tomarked is subject to constant change. Even if the body 26 were to bestationary when marked, if the desired mark were of a sufficient sizeany curvature of the body 26 would also give rise to differing distancesbetween the lens assembly 76 and various points on the surface. On topof this, successive bodies to be marked may be placed on the conveyorbelt 16 at different distances from the fixed side rail 20 despite anarrowing of the effective width of the conveyor belt 16 prior to thelaser marking station. If, as has been described, the first lens element78 is of a fixed focal length, each of the above factors will contributeto parts of the mark applied to the body being more or less out offocus. However, with the careful selection of the focal length of thelens element 78 this problem maybe reduced to a minimum.

As has been previously stated, the focal length of the first lenselement 78 is typically between 70 mm and 80 mm and is capable offocusing the combined CO₂ /He-Ne beam 46, 58 so as to yield at the focusa power density that is typically in excess of 300 W/cm². Having saidthat however, for a lens element having a focal length within thisrange, the power density at a small distance δx from the focus is stillsufficient to cause thermal interactions to occur within the body to bemarked. In a preferred embodiment, the lens element 78 has a focallength of 75 mm, enabling δx for glass to be as large as 5 mm, althoughthe size of δx is, of course, dependant upon the material of which thebody 26 is comprised. Using such a lens however, the apparatus describedmay efficiently mark moving bodies whose surfaces lie within a smallrange of distances either side of an optimum distance from the lensassembly 76.

Alternatively, or in addition, a second lens element 92 may be placed inseries with the first lens element 78 in order to compensate for one ormore of the de-focusing effects described above. Such a lens element 92may posess a focal length that varies across its width and may, forexample, comprise a flat field lens so as to compensate for anycurvature of the surface of the body to be marked.

In another arrangement, the lens assembly 76 may include a third lenselement 94 in the form of a zoom lens whose focal length may be alteredas the body to be marked passes the marking apparatus 10, therebymaintaining the focus of the combined CO₂ /He-Ne Beam 46, 58 at thedesired point on the surface of the body 26 in spite of the de-focusingeffects described above.

In yet a further arrangement, in place of the second lens element 92 andor the third lens element 94 there may be disposed a fourth lens element95 taking the form of a diverging lens. The fourth lens element 95 offocal length f₂ is preferably disposed a distance f₂ in front of thefocus that would otherwise be produced by the first lens element 78. Inthis way, the fourth lens element 95 produces a narrow parallel beam ofhigh energy density radiation which may be directed at the moving body26 to produce an illuminated spot on the surface thereof. Provided thatthe narrow beam has a sufficient power density, it may be used tofacilitate the surface marking of the moving body 26 while at the sametime not being prone to any of the de-focusing effects described above.

In a second embodiment shown in FIG. 6, the marking apparatus 10 isagain designed to facilitate the surface marking of a moving body ofmaterial 26, except that rather than superimposing a component thatcompensates for that movement on the already complex movement of thefirst and second mirrors 68 and 70, the movement of the body 26 isentirely compensated for by a fifth reflecting surface 96.

The fifth reflecting surface 96 is rotatably mounted about an axis 98and is positioned so as to direct onto the moving body 26 the combinedCO₂ /He-Ne beam 46, 58 that is incident upon it as a result of areflection from the second mirror 70. As the body to be marked passesthe marking apparatus 10, the fifth reflecting surface 96 rotates aboutthe axis 98 in such a way as to keep the combined CO₂ /He-Ne beam 46, 58directed upon the moving body 26.

The fifth reflecting surface 96 preferably comprises the mirror of athird galavanometer 100. In this way the movement of the fifthreflecting surface 96 may be facilitated with the same speed of responseand ease of control as enjoyed by the first and second mirrors 68 and70. Under such circumstances, when the CO₂ laser 44 is actuated and the15 v DC supply, applied to the first and second galvanometers 72 and 74,is modulated to produce the pre-determined movement of the first andsecond mirrors 68 and 70, a separate 15 v DC supply may be applied tothe third galvanometer 100 and modulated in accordance with thepreviously measured velocity characteristic of the moving body 26. Asbefore, the combined effect of the movement of the mirrors of the threegalvanometers 72, 74 and 100 is to enable the real time dynamic scanningof the moving body 26 by the combined CO₂ /He-Ne beam 46, 58.

In FIG. 6 the fifth reflecting surface 96 is shown disposed between thesecond mirror 70 and the lens assembly 76, although it will be apparentto those skilled in the art that the fifth reflecting surface 96 mayequally well be disposed at other points along the optical path of thecombined CO₂ /He-Ne beam 46, 58, such as, for example, immediately afterthe lens assembly 76.

In a third embodiment which is similar to the second in that thecompensation for the movement of the body 26 is made separately from thegeneration of the mark itself, the fifth reflecting surface 96 isreplaced by a multi-faceted mirror 102 as shown in FIG. 7. As with thefifth reflecting surface 96, the multi-faceted mirror 102 is rotatablymounted about an axis 104 and positioned so as to direct onto the movingbody 26 the combined CO₂ /He-Ne beam 46, 58 that is incident upon it asa result of a reflection from the second mirror 70. As the body to bemarked passes the marking apparatus 10, the multi-faceted mirror 102rotates about the axis 104 in such a way as to keep the combined CO₂/He-Ne beam 46, 58 directed upon the moving body 26.

The advantage of this third embodiment, as distinct from the secondembodiment described above, is that once the moving body 26 has beenmarked, the multi-faceted mirror 102, unlike the fifth reflectingsurface 96 of the second embodiment, does not need to rotate rapidlyabout the axis 104 in either sense in order to be appropriately alignedfor the next body to be marked. Instead, the multi-faceted mirror 102may continue to rotate in the same sense and at such a speed as toenable the combined CO₂ /He-Ne beam 46, 58 to be directed onto the nextbody to be marked by virtue of a reflection from a different surface ofthe multi-faceted mirror 102. The shape of the multi-faceted mirror 102does, however, impose conditions on its own rotational speed, which mustbe such as to ensure that it does not rotate through an angle greaterthan that subtended by the operative face during the time taken to markthe moving body 26.

The rotation of the multi-faceted mirror 102 may be controlled by thecomputer 90 once the velocity of the moving body 26 has been measuredand the numher of vectors required to trace out the desired mark isknown, since the latter enables a prediction of the necessary markingtime, while the former permits a calculation of the distance the body 26will be transported while being marked.

In FIG. 7 the multi-faceted mirror 102 is shown disposed between thesecond mirror 70 and the lens assembly 76, although it will be apparentto those skilled in the art that the multi-faceted mirror 102 mayequally well be disposed at other points along the optical path of thecombined CO₂ /He-Ne beam 46, 58, such as, for example, immediately afterthe lens assembly 76.

In a fourth embodiment of the marking apparatus 10 shown in FIG. 8, themovement of the body 26 is compensated for by a lateral movement of theentire head unit 64 and lens assembly 76. Having measured the velocityof the body to be marked, the head unit 64 and lens assembly 76 is movedin a direction parallel to the moving body 26 under the action of amotor (not shown). By moving the head unit 64 and lens assembly 76 atthe same velocity as the moving body 26, the relative velocity betweenthe two may be reduced to zero while the desired mark is applied. Oncethe moving body 26 has been marked, the head unit 64 and lens assembly76 are rapidly returned to their starting positions, again under theaction of the motor, so as to be ready for the next body to be marked.

By ensuring that the combined CO₂ /He-Ne beam 46, 58 that is reflectedfrom the first mirror 68 travels in a direction parallel to the conveyorbelt 16 before being reflected toward the moving body 26 at the secondmirror 70, it will be apparent to those skilled in the art that only thesecond mirror 70 and the lens assembly 76 need be moved by the motor inorder to achieve the desired effect. Indeed, if the lens assembly 76were disposed in the optical path of the combined CO₂ /He-Ne beam 46, 58between the fourth reflecting surface 60 and the first mirror 68, thenonly the second mirror 70 would need to be moved by the motor.

In a fifth embodiment depicted in FIG. 9, one or more acusto-optic orelctro-optic crystals 108 may be disposed in the path of the beam 46, 58to compensate for the movement of the body 26. Crystals of these typespossess the property of being able to deflect an incident beam throughdifferent angles, depending on the value of a voltage applied thereto.Therefore, by applying a suitably varying voltage to the crystals 108,the combined CO₂ /He-Ne beam 46, 58 may continue to be directed at themoving body 26 as it passes the marking apparatus 10.

It will also be apparent to those skilled in the art that in the lightof the Applicant's co-pending UK Patent Application No. 9117521.6 theapparatus described in relation to any of the foregoing embodiments mayalso be employed to facilitate the sub-surface marking of a moving bodyof material without substantial alteration.

In the past, in order to produce an indelible mark, manufacturers haverelied almost exclusively on surface marking. However, one of thefundamental problems with this type of marking is that it may either bedestroyed by removing a part of the surface upon which the mark isapplied or imitated by the application of an identical mark on asubstitute body. By use of apparatus similar to that already described,a moving body of material may be provided with a sub-surface mark bydirecting at the surface of the body a focused beam of high energydensity laser radiation to which the material is transparent. The beamis focused at a location spaced from the surface and within the body soas to cause localised ionisation of the material and the creation of amark in the form of an area of increased opacity to electromagneticradiation substantially without any detectable change at the surface.

For the avoidance of doubt, the term transparent as used above withreference to the material to be marked refers to a material in which thehigh energy density beam can penetrate at least to the depth of thedesired mark and as such includes translucent materials and materialssuch as coloured or smoked glass in which the transmissioncharacteristic to electromagnetic radiation at wavelengths in thevisible region has been reduced but not eliminated. The term transparentalso includes materials which are opaque to electromagnetic radiation atwavelengths in the visible region but which are at least capable oftransmitting electromagnetic radiation at wavelengths within the sameregion of the electromagnetic spectrum as that of the high energydensity beam.

The possible types of interaction between laser radiation and a body ofmaterial may be categorised under three headings dependent upon thepower density of the laser radiation concerned. In order of increasingpower density these headings are as follows:

1. Photochemical interactions including photoinduction andphotoactivation;

2. Thermal interactions in which the incident radiation is absorbed asheat; and

3. Ionising interactions which involve the non-thermalphotodecomposition of the irradiated material.

The difference between the thresholds of these three interactions isclearly demonstrated by comparing the typical power density of 10⁻³W/cm² required to produce a photochemical interaction with the powerdensity of 10¹² W/cm² typical of ionisation interactions such asphotoablation and photodisruption.

For localised ionisation of the material to take place, the high energydensity beam must posess sufficient energy to cleave molecular bonds andcreate a plasma at the point of focus. Once the beam has been removed,the plasma cools to form a localised zone of damage or disruption whichscatters any elctromagnetic radiation that is incident upon it, with theresult that the zone appears as an area of increased opacity.

At present, the only commercially available lasers capable of inducingionisation interactions are pulsed lasers having a peak energy that,when focused, is sufficient to create a plasma within the materialconcerned. In order to facilitate the sub-surface marking of a movingbody, therefore, the source 44 of laser radiation is preferably replacedby a laser having a power density at its focus of at least 10⁷ W/cm² anda pulse duration of no more than 10⁻⁶ seconds. In this way the energydensity of each pulse is at least 10 J/cm² and is sufficient to inducelocalised ionisation of the material at the focus of the beam.

If the sub-surface mark is to be visible to the naked eye, the body tobe marked must be transparent to electromagnetic radiation atwavelengths within the visible region. For example, the body may be ofglass or plastic. The body to be marked, however, need not necessarilybe limited in this way and may comprise a material that is opaque toelectromagnetic radiation at wavelengths within the visible region.Under these circumstances the resulting sub-surface mark is hidden tothe naked eye but may be "seen" by optical instruments operating at anappropriate wavelength within the electromagnetic spectrum such as thatof the high energy density beam. While such a mark is not capable ofperforming many of the functions of its visible counterpart, it doesrepresent a truly indelible covert mark.

Assuming that the eventual sub-surface mark is intended to be visible tothe naked eye and that, therefore, the moving body 26 is of a materialsuch as glass or plastic that is transparent to electromagneticradiation within the visible region of the elctromagnetic spectrum, thesource 48, in addition to the power constraints identified above, mustalso be selected so that the material of the body 26 is transparent tothe laser radiation 50 that it produces. Under these circumstances thesource 48 preferably comprises a Nd-YAG (Neodymium-doped YttriumAluminium Garnet) laser operating at a wavelength of 1.06 μm.

The remainder of the apparatus described need not be substantiallyaltered in order to facilitate sub-surface marking, although theselection of the source 44 will of course have an effect on the choiceof the optical elements used to direct and focus the resulting laserradiating, since not all such elements will operate with the sameefficiency at different wavelengths within the electromagnetic spectrum.It is, however, considered that the appropriate selection of elementsfalls within the ordinary expertise of one skilled in the art.

When employed to facilitate the sub-surface marking of a moving body,the lens assembly 76 may include a third lens element 94 in the form ofa zoom lens so that marks maybe made at different depths within themoving body 26 and so to allow for the creation of three dimensionalmarks.

It will be apparent to those skilled in the art that while the apparatusdescribed includes means for determining the velocity of the moving body26, this need not necessarily be so, since a mechanical linkage may beincorporated into the apparatus that imparts to the combined CO₂ /He-Nebeam 46, 58 a component of movement equal to the velocity of the movingbody 26 without ever determining what that velocity is.

We claim:
 1. A method of marking a succession of bodies moving along a predetermined path, said method comprising the steps of:(a) activating a single detector unit to detect the presence and the movement of one of the moving bodies at a predetermined location along said path; (b) in response to detection of the movement of said one of the bodies, determining the velocity of said one of the bodies; (c) activating an energy source at a calculated time following detection of the presence of said one of the bodies to direct a high energy density beam at said one of the bodies as said one of the bodies moves along said path, the calculated time being dependent upon the determined velocity; (d) concentrating the beam so as to produce an illuminated spot at a location on or within said one of the bodies; (e) moving the high energy density beam so as to move said spot; and (f) controlling the movement of the high energy density beam so as to control the movement of said spot in accordance with the resultant of two components of spot movement, the first component being equal to the determined velocity, and the second component being relative to said one of the bodies, so as to create a mark of a predetermined shape at a desired location on or within said one of the bodies.
 2. A method in accordance with claim 1, wherein the velocity of said one of the bodies is determined by direct measurement.
 3. A method in accordance with claim 1, wherein step (c) comprises causing the high energy density beam to intersect the path of said one of the bodies, and activating the energy source at a predetermined time after said one of the bodies passes a position a known distance from the intersection, the predetermined time being dependent upon the determined velocity of said one of the bodies.
 4. A method in accordance with claim 1, wherein step (d) comprises bringing the high energy density beam to a focus at a location within said one of the bodies so as to cause localised ionisation of the material of which said one of the bodies is formed and creation of a mark in the form of an area of increased opacity to electromagnetic radiation.
 5. A method in accordance with claim 4, wherein said one of the bodies is transparent to electromagnetic radiation at wavelengths within the visible region.
 6. A method in accordance with claim 4, wherein said one of the bodies is opaque to electromagnetic radiation at wavelengths within the visible region.
 7. A method in accordance with claim 1, wherein the mark comprises one or more numerals, letters, or symbols, or a combination thereof.
 8. A method in accordance with claim 1, wherein the mark is three dimensional.
 9. An apparatus for marking a succession of bodies moving along a predetermined path, said apparatus comprising:a single detector unit for detecting the presence and determining the velocity of one of the bodies at a predetermined location along said path; an energy source; a first controller for activating the energy source at a calculated time after said single detector unit has detected the presence of said one of the bodies, to create a high energy density beam, and directing the beam at said one of the bodies, the calculated time being dependent upon the determined velocity; a beam concentrator for concentrating the beam so as to produce an illuminated spot at a location on or within said one of the bodies; a beam moving system for moving the high energy density beam so as to move said spot; and a second controller for controlling said beam moving system to provide controlled movement of the spot in accordance with the resultant of two components of movement, the first component being equal to the determined velocity, and the second component being relative to said one of the bodies, so as to create a mark of a predetermined shape at a desired location on or within said one of the bodies.
 10. An apparatus in accordance with claim 9, wherein said beam moving system moves the spot in accordance with said second of the two components of movement and comprises at least one moveable mirror disposed in the path of the beam.
 11. An apparatus in accordance with claim 10, wherein the movement of the said at least one moveable mirror is controlled in accordance with a computer program.
 12. An apparatus in accordance with claim 10, wherein the said at least one moveable mirror is a galvanometer mirror.
 13. An apparatus in accordance with claim 10, wherein said beam moving system also moves the spot in accordance with said first of the two components of movement.
 14. An apparatus in accordance with claim 10, wherein said beam moving system includes a beam moving unit for moving the spot in accordance with said first of the two component of movement.
 15. An apparatus in accordance with claim 14, wherein said beam moving unit includes at least one rotatably mounted mirror whose speed of rotation is varied in accordance with the velocity of said one of the bodies.
 16. An apparatus in accordance with claim 15, wherein said at least one rotatably mounted mirror is a multi-faceted mirror.
 17. An apparatus in accordance with claim 14, wherein said beam moving unit is responsive to the determined velocity to move said at least one mirror at a velocity causing the spot to move at the same velocity as said one of the bodies.
 18. An apparatus in accordance with claim 14, wherein said beam moving unit includes at least one acousto-optic or electro-optic crystal.
 19. An apparatus in accordance with claim 9, wherein said single detector unit includes two opto-detector devices spaced a known distance apart, and a timer for measuring the time taken for said one of the bodies to travel between said two opto-detector devices.
 20. An apparatus in accordance with claim 9, wherein said beam concentrator comprises a lens assembly for directing the high energy density beam at the path of said one of the bodies, causing the beam to intersect the path when said energy source is activated, and a third controller for activating the energy source at a predetermined time after said one of the bodies passes a position a known distance from the intersection, the predetermined time being dependent upon the velocity of said one of the bodies.
 21. An apparatus in accordance with claim 9, wherein said beam concentrator includes a lens element having a focal length that varies across its width.
 22. An apparatus in accordance with claim 9, wherein said beam concentrator includes a zoom lens.
 23. An apparatus in accordance with claim 9, wherein said beam concentrator includes a diverging lens.
 24. An apparatus in accordance with claim 9, wherein said beam concentrator produces the illuminated spot on said one of the bodies so as to create the mark as a surface mark.
 25. An apparatus in accordance with claim 24, wherein said energy source comprises a CO₂ laser.
 26. An apparatus in accordance with claim 9, wherein said beam concentrator produces the illuminated spot within said one of the bodies so as to create the mark as a sub-surface mark.
 27. An apparatus in accordance with claim 26, wherein said energy source comprises a laser focused so as to have a peak energy density at the focus of at least 10 J/cm².
 28. An apparatus in accordance with claim 26, wherein said energy source comprises a laser focused so as to have a power density at the focus of at least 10⁷ W/cm² and a third controller for pulsing the laser with a pulse duration of at least 10⁻⁶ seconds.
 29. An apparatus in accordance with claim 26, wherein said energy source comprises a Nd-YAG laser.
 30. An apparatus in accordance with claim 9, further comprising a secondary source of visible laser radiation to facilitate alignment of the high energy density beam.
 31. An apparatus in accordance with claim 9, further comprising a conveyor belt to transport said one of the bodies along the predetermined path.
 32. An apparatus in accordance with claim 31, further comprising a position controller to control the lateral position of said one of the bodies relative to the conveyor belt. 